Process for etching aluminum alloy surfaces



United States atent T PROCESS FOR ETCHING ALUMINUM ALLOY SURFACES Kenneth Walter Newman, Burbank, Don C. Atkins,;Jr., Whittier, and Gordon S. Douglas, Los Angeles, Calif., assignors to Turco Products, Inc., Los Angeles, Calif., a corporation of California No Drawing. Application January 31, 1955,

Serial No. 485,304

14 Claims. (CI. 41-42) solution. The etched article is then further treated to remove smut, e. g., by immersion in nitric acid, washed, and dried.

When aluminum alloys are etched in alkali solutions, e. g., containing sodium hydroxide, potassium hydroxide, or equivalent agents as alkali, under some conditions of treatment, such as temperature and concentration of the etching solution, and with some alloys, e. g., zinc and copper alloys of aluminum, a relatively pebbly or nodular etched surface is obtained. This may occur when such alkali etching solutions contain the alkali material as sole ingredient, or when the solution also contains additives such as citric acid or salts thereof, as described and claimed in application Serial No. 445,197, filed July 22, 1954, now U. S. Patent No. 2,739,883, by Kenneth W. Newman; or other agents such as, for example, gluconic acid, or salts thereof, sorbitol, and the like. An etching bath producing a nodular etched surface as aforementioned is termed herein a nodulizing etching solution. The theory of why this occurs is not known to applicants.

However, many applications of etched aluminum alloys require that the etched surface be smoother and 'less nodular than is obtained under the above conditions.

One object of this invention is to provide etched surfaces of aluminum alloys having improved appearance.

Another object is to provide procedure for producing etched alurninum alloy surfaces which are smooth and substantially free of bumps and nodules.

Yet another object is the provision of relatively simple and inexpensive procedure for obtaining improvedetched surfaces on aluminum alloys, particularly copper and zinc alloys of aluminum.

We have discovered that when the etched surface of an aluminum alloy treated in an alkali etching solution is bumpy and nodular, i. e. the solution is a nodulizing etching solution, if a normal alkali metal sulfide, an alkali metal polysulfide, or sulfur is added to this nodulizing etching solution and a piece of said alloy is treated inthe resulting solution, the etched surface so obtained is substantially free of bumps or nodules, and is smoother and more uniform than that produced in the nodulizing-etching solution in the absence of these sulfur-containing materials. Examples of alkali metal sulfides :and polysulfides which can be employed are sodium sulfide, potassium sulfide, sodium polysulfide, and potassium polysulfide. The preferred additive according to the invention :isnor- ,ing etching solution.

2,795, 03 Patented June 11, 1057 mal sodium sulfide (NazS). These materials may be employed separately, or mixtures of two or more of these materials can be used.

Examples of aluminum alloys which can be treated to form a smooth etched surface according to the invention are as follows:

Designation Approximate Composition 148---. 0.501.2% Si, 1.0% Fe 3.95.0% Cu, DAG-1.2% Mn, 0.20-0.s% Mg, 0.10% 6r, 0.25% Zn, 0.15% Ti, Rem. A1. 178 0.8% Si, 1.0% Fe, 3.54.5% Cu, 0.40l.0% M11, 0.200.8%

Mg, 0.10% Cr, 0.25% Zn Rem. A1. 248 0.50% Si, 0.50% Fe, 38-49% Cu, 0.300.9% Mn, 1.21.8%

Mg, 0.10% Cr, 0.25% Zn, Rem. Al. 61S 0.400.8% Si, 0.7% Fe, 0.150.40% C11, 0.15% Mn, 0.8-

1.2% Mg, 0.l50.35 Cr, 0.25% Zn, 0.15% Ti, Rem. A1. 758 0.50% Si, 0.7% Fe, 12-20% Cu, 0.30% Mn, 2.12.9% Mg, 0.180.40% Cr, 5.l6.1% Zn, 0.20% Ti, Rem. Al.

Other aluminum alloys may also be etched employing the invention principles, to obtain the invention results.

-In'-our copending application Serial No. 485,303, filed of even date herewith, we have described the use of alkali metalsulfides and polysulfides, and sulfur in alkali solutions for etching aluminum alloys.

The etching treatment is carried out in a hot aqueous solution containing an alkali such as sodium or potassiumhydroxide, trisodium phosphate, sodium carbonate, or the like, or mixtures thereof. Temperature of the solution may range say from room temperature, for example fide, or sulfur added to the nodulizing etching solution may likewise vary but is usually substantially less than the amount of alkali etchant employed. Generally, from about -'0.1 to about 10% of the sulfur-containing additive based on the weight of the alkali can be used. Amounts of the additive in the range of about 2 to about 6% based on the weight of the alkali etchant are preferred, with an amount of about 3% most desirable. While the use of a quantity of alkali metal sulfide, e. g., sodium sulfide, polysulfide, or sulfur, in excess of 3% by weight of the alkali etchant provides some enchancement in smoothing action or removal of nodules from the etched aluminum alloy surface, the use of substantially more than 3% of the ad- .ditive is not economical in relation to the degree of improvement obtained thereby. The sulfur-containing additive in the solution can be replenished from time to time where the bath is in continuous use.

In practicing the invention, a test piece of the aluminum alloy to be etched is immersed in the alkali etching solution and etched for a period sufficient to dissolve a substantial amount of surface metal, e. g. to inch, un-

der the conditions, e. g., concentration of solution and temperature, at which a part composed of the same alloy is to be etched. For example, the alkali etchant solution concentration may be 5% or 10%, and the solution tem- -perature may be F. 'The test piece is then withdrawn, the smut removed, e. g., by a nitric acid solution, and the sample washed and dried. If the etched surface .so obtained is bumpy or nodular, the solution is a noduliz- A sulfur-containing additive according to the invention is then incorporated in the aforementioned amounts in the nodulizing etching solution. The-part to be etched is then immersed in the resulting solution .for the period required to produce the desired depth ofetch on the surface of the part.

.The,.dark highly adherent film called smut generally acids, and generally require treatment in nitric acid, mixtures of HNO: and H2504, or mixtures of chromic and sulfuric acids for their removal. Examples of smut removing compositions which can be used are a to solution of nitric acid, and a solution of the composition disclosed in application Serial No. 448,775, filed August subsequent washing and drying of the aluminum alloy surface of the part etched according to the invention, such surface is found to be very smooth and uniform, and substantially free of nodules.

perature of 190 F. except that in one case the solution contains our sulfur-bearing material.

The following are examples of practice of the invention, all quantities where not otherwise specified being in terms of parts by weight.

Example I I A test piece of 24S aluminum alloy was immersed in a 5% caustic soda etching solution and etched therein at a solution temperature of 190 F. until the exposed surface was etched to a depth of about inch. The article was then withdrawn from the bath, the smut removed from the etched surface by treatment witha solution of the smut removing composition of application Serial No. 448,775, described above, and the article washed and dried. The etched surface so obtained was observed to be coarse grained and nodular, and composed of bumps or hillocks of relatively large size. This indicated the above solution was a nodulizing etching solution.

When using a nodulizing caustic soda etching solution or a nodulizing etching solution containing an equivalent alkaline etching agent, free however of our sulfur bearing materials, the result is a'rough pebbly etched surface as aforementioned. Such surface appears full of small Another aqueous etching solution was prepared by dissolving an etchant composition in water to produce a solution concentration of 5%. The etchant composition was composed of 97 parts of caustic soda and 3 parts of sodium sulfide. An article having a 248 aluminum alloy surface was etched in this solution while maintained at a temperature of 190 F. The article was then bumps easily discernible with the naked eye. The bumps are distributed over the surface in a heterogeneous irregular pattern with valleys between the bumps. It is believed these bumps are the result of an uneven rate of etch across the entire surface, the etch rate being least at the apices of the bumps and increasing along the surfaces, moving from the tops of the bumps into the valleys. The surface is shiny under strong reflected sunlight, and ap pears visually as illuminated across the surface with no interference or refraction phenomenon since no color is discernible.

However, if as little as 1% of the sulfur-containing material of the invention is added to the nodulizing etching solution, the etched surface obtained is smooth and while a surface etch pattern is visible in the form of barely discernible striations, no discrete hillocks are observable even with a magnifying 2 power lens. The surface appears crystalline but smooth under strong reflective sunlight and iridescent and colored, indicating either interference or refraction phenomenon. This surface may be termed jewel-like for this reason.

The difference in surface, i. e., bumpy, large grained, and irregular in the case of the nodulizing alkali etchant solution free of sulfur-containing material, and the smooth, fine grained, non-bumpy surface when employing such additive material in said solution is readily discernible in any form of illumination or even by feel. The etched surface obtained when using the nodulizing etchant solution free of the sulfur-bearing material of the invention will be termed irregular or coarse grained, as well as nodular, and when such sulfur-containing material is used in said solution, smooth or fine grained.

It is to be understood that comparisons of the coarse grained nodular surface obtained on an aluminum alloy under nodulizing conditions, with the substantially nodule free fine grained surface produced by use of our sulfurbearing materials, e. g., sodium sulfide, according to the invention, is based on the use of the same etching conditions, i. e., the same alkali etchant such as caustic soda, the same solution concentration and the same temperature, and also on the use of the same aluminum alloy, except that the etching solution in one case wherein the smooth, fine grained surface is obtained also contains the sulfur-bearing material. Thus, for example, the etched surfaces of two aluminum alloy parts of the same alloy composition can be compared for the invention purposes by etching each for the same period of time in the same 5% caustic soda etching solution and at the same temwithdrawn from the bath, the smut removed from the etched surface by treatment with a solution of the smut removing composition of application Serial No. 448,775,

described above, and the article washed and dried. The etched aluminum alloy surface so obtained was fine grained and in the form of striations with no discernible nodules or hillocks.

Example 2 composition was composed of 99 parts of NaOH and 1 part of sodium sulfide. The results obtained were similar to those obtained in Example 1.

Example 3 An aqueous etching solution was prepared by dissolving an etchant composition in Water to produce a solution concentration of 10%. The etchant composition was composed of 96 parts of caustic soda and 4 parts of sodium sulfide. An article having a 245 aluminum alloy surface was etched in this solution while maintained at a temperature of F. The article was then withdrawn from the bath, the smut removed from the etched surface of the article by treatment with a solution of the smut removing-composition of application Serial No. 448,775, described above, and the article washed and dried. The etched aluminum alloy surface of the article was fine grained, smooth, and substantially free of nodules.

Example 4 An aqueous etching solution was prepared by dissolving an etchant composition in water to produce a solution concentration of 10%. The etchant composition was composed of 96 parts of caustic soda and 4 parts of sulfur in the form of flowers of sulfur. A 248 aluminum alloy was etched in the solution while maintained at 190 F. Following smut removal, as previously described, and washing, the etched surface of the alloy was observed to be smooth, nodule-free, and uniform.

Example 5 Results similar to Example 4 may be obtained by employing the procedure and conditions of Example 4, but substituting sodium polysulfide for the sulfur.

Example 6 The same procedure was carried out as in Example 4,

76 except that the aluminum alloy surface etched was composed of 75S alloy instead of 248. The results obtained were similar to those obtained in Example 4.

While we have described a particular embodiment of our invention for the purpose of illustration, it should be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

We claim:

1. A method of etching an article having an aluminum alloy surface to produce a smooth substantially nodulefree etched surface, which comprises treating said alloy surface with an alkali etching solution having dissolved therein a material of the group consisting of an alkali metal sulfide, an alkali metal polysulfide, and sulfur.

2. A method of etching an article having an aluminum alloy surface to produce a smooth substantially nodulefree etched surface, which comprises treating said alloy surface with a hot alkali etching solution containing caustic soda and having dissolved therein a material of the group consisting of an alkali metal sulfide, an alkali metal polysulfide, and sulfur.

3. A method of etching an article having an aluminum alloy surface to produce a smooth substantially nodulefree etched surface, which comprises treating said alloy surface with an alkali etching solution having dissolved therein an alkali metal sulfide.

4. A method of etching an article having an aluminum alloy surface to produce a smooth substantially nodulefree etched surface, which comprises treating said alloy surface with an alkali etching solution having dissolved therein sulfur.

5. A method of etching an article having an aluminum alloy surface to produce a smooth substantially nodulefree etched surface, which comprises treating said alloy surface with an alkali etching solution having dissolved therein an alkali metal polysulfide.

6. A method of etching an article having an aluminum alloy surface to produce a smooth substantially nodulefree etched surface, which comprises treating said alloy surface with an alkali etching solution having dissolved therein sodium sulfide.

7. A method of etching an article having an aluminum alloy surface to produce a smooth substantially nodulefree etched surface, which comprises treating said alloy surface with a hot alkali etching solution having dissolved therein from about 0.1 to about by weight of said alkali, of sodium sulfide.

8. A method of etching an article having an aluminum alloy surface to produce a smooth substantially nodulefree etched surface, which comprises treating said alloy surface with a hot alkali etching solution containing caustic soda and having dissolved therein from about 2 to about 6% by weight of said caustic soda, of sodium sulfide.

9. A method of etching an article having an aluminum alloy surface to produce a smooth substantially nodulefree etched surface, which comprises treating said alloy surface with a hot alkali etching solution having dissolved therein from about 0.1 to about 10% by weight of said alkali, of sulfur.

10. A method of etching an article having an aluminum alloy surface, which comprises adding a material of the group consisting of an alkali metal sulfide, an alkali metal polysulfide, and sulfur, to a nodulizing alkali etching solution, and treating said alloy surface in the resulting solution to obtain a smooth substantially nodulefree etched surface.

11. A method of etching an article having an aluminum alloy surface, which comprises adding a material of the group consisting of an alkali metal sulfide, an alkali metal polysulfide, and sulfur to a nodulizing hot alkali etching solution containing caustic soda, and treating said alloy surface in the resulting solution to obtain a smooth substantially nodule-free etched surface.

12. A method of etching an article having an aluminum alloy surface, which comprises adding sodium sulfide to a nodulizing hot alkali etching solution containing caustic soda, and treating said alloy surface in the resulting solution to obtain a smooth substantially nodule-free etched surface, the amount of sodium sulfide added being about 0.1 to about 10% by weight of said caustic soda.

13. A method of etching an article having an aluminum alloy surface, which comprises adding sulfur to a nodulizing hot alkali etching solution containing caustic soda, and treating said alloy surface in the resulting solution to obtain a smooth substantially nodule-free etched surface, the amount of sulfur added being about 0.1 to about 10% by weight of said caustic soda.

14. A method of etching an article having an aluminum alloy surface, which comprises adding sodium polysulfide to a nodulizing hot alkali etching solution containing caustic soda, and treating said alloy surface in the resulting solution to obtain a smooth substantially nodulefree etched surface, the amount of sodium polysulfide added being about 0.1 to about 10% by weight of said caustic soda.

References Cited in the file of this patent UNITED STATES PATENTS 2,739,883 Newman Mar. 27, 1956 

1. A METHOD OF ETCHING AN ARTICLE HAVING AN ALUNIMUM ALLOY SURFACE TO PRODUCE A SMOOTH SUBSTANTIALLY NODULEFREE ETCHED SURFACE, WHICH COMPRISES TREATING SAID ALLOY SURFACE WITH AN ALKALI ETCHING SOLUTION HAVING DISSOLVED THEREIN A MATERIAL OF THE GROUP CONSISTING OF AN ALKALI METAL SULFIDE, AN ALKALI METAL POLYSULFIDE, AND SULFUR. 